• Adjust racks to top and middle positions and preheat oven to 450 degrees. Wash and dry produce. • Dice potatoes into ½-inch pieces. Trim and discard woody bottom ends from asparagus. Pick and finely chop fronds from dill.
• Toss potatoes on a baking sheet with a drizzle of oil and a pinch of salt and pepper. • Roast on top rack until lightly browned and tender, 20-25 minutes.
• Meanwhile, pat chicken* dry with paper towels; season all over with salt and pepper. • Heat a drizzle of oil in a large pan over medium-high heat. Add chicken and cook until browned and cooked through, 3-5 minutes per side. • Turn off heat; transfer chicken to a cutting board to rest. (Keep pan handy for step 5.)
• While chicken cooks, toss asparagus on a second baking sheet with a drizzle of oil and a pinch of salt and pepper. • Roast on middle rack until tender, 10-12 minutes.
Meanwhile, heat pan used for chicken over medium heat. Stir in stock concentrate, 2 TBSP water (4 TBSP for 4 servings), and as much chopped dill and mustard as you like. (Save the rest of the chopped dill for serving.) • Remove pan from heat; stir in sour cream and 1 TBSP butter (2 TBSP for 4), scraping up any browned bits from bottom of pan, until melted and combined. Season with salt and pepper. TIP: If sauce seems too thick, add more water 1 tsp at a time until it reaches a drizzling consistency.
• Slice chicken crosswise. • Divide chicken, potatoes, and asparagus between plates. Drizzle pan sauce over chicken. Garnish with any remaining chopped dill to taste and serve.